The first cellular mobile radio systems in public use were generally analog systems for the transmission of speech or other analog information. The systems comprised a plurality of radio channels for transmitting analog information between base and mobile stations by transmitting analog modulated radio signals. In general, the first cellular mobile radio systems had comparably large coverage cells. More recently, digital cellular mobile radio systems for public use have been designed.
Digital cellular mobile radio systems comprise digital channels for transmitting digital or digitized analog information between base and mobile stations, by transmitting digitally modulated radio signals. Digital cellular mobile radio systems offer substantial advantages over analog cellular mobile radio systems.
One digital mobile radio system intended to be used as a common system for many European countries is the GSM system. In European countries already having an analog cellular mobile system, the new digital GSM system is intended to be introduced as a new system which is independent of any existing analog system. The GSM system base and mobile stations have not been designed to be compatible with existing systems, however, they are designed to give optimum performances in various aspects in and of the system itself. Accordingly, there has been a comparatively great freedom of choice in technical matters when designing the GSM system.
Rather than introduce a new independent digital cellular mobile radio system, like the GSM system, in an area with an existing analog cellular system, it has been proposed to introduce a digital cellular mobile radio system which is designed for cooperation with the existing analog cellular mobile radio system. In order to obtain digital channels within the frequency band allotted to cellular mobile radio systems, there have been proposals to withdraw a number of radio channels allotted to the present analog mobile radio systems and use them in the digital cellular mobile radio system. Due to the proposed design of the digital mobile radio system, three or possibly six digital channels may occupy the same frequency band of one previous analog radio channel by using time division multiplexing. Accordingly, replacing some analog channels by digital channels in time division multiplex may increase the total number of channels.
The intended result is to gradually introduce the digital system and to increase the number of digital traffic channels while decreasing the number of analog traffic channels in the coexisting cellular systems. Analog mobile stations already in use will then be able to continue to use the remaining analog traffic channels. Meanwhile, new digital mobile stations will be able to use the new digital traffic channels. Dual-mode mobile stations will be able to use both the remaining analog and the new digital traffic channels.
With the addition of the new digital traffic channels, a corresponding need for new digital control channels arises. The conventional dual-mode systems for the most part utilize existing analog channels, such as dedicated frequencies, as the control channel.